Methods are being sought for transferring functional genes into mammalian tissue culture cells and ultimately into intact animals. A primary technique is physical microinjection of specific genes into the nucleus of individual tissue culture cells. A genetic defect, lack of thymidine kinase (TK) activity, was corrected in mouse TK-L cells by microinjecting a herpes simplex TK gene. In addition, co-injected human beta globin gene was replicated and expressed at a low level in these cells.